Like millions of motorists, Eric Hanson used a Global Positioning System
device in his Chevrolet TrailBlazer to find his way around. He probably
did not expect that prosecutors would use it, too — to help convict him
of killing four family members.

Prosecutors in suburban Chicago analyzed data from the Garmin G.P.S.
device to pinpoint where Mr. Hanson had been on the morning after his parents
were fatally shot and his sister and brother-in-law bludgeoned to death
in 2005. He was convicted of the killings this year and sentenced to death.

Mr. Hanson’s trial was among recent criminal cases in which the authorities
used such navigation devices to help establish a defendant’s whereabouts.
Experts say such evidence will almost certainly become more common in court
as the systems become more affordable and show up in more vehicles.

“There’s no real doubt,” said Alan Brill, a computer forensics expert
in Minnesota who has worked with the Federal Bureau of Investigation and
the Secret Service. “This follows every other technology that turns out
to have information of forensic value. I think what we’re seeing is evolutionary.”

Using technology to track a person’s location is nothing new, but the
popularity of the Global Positioning System — in cars, cellphones and other
handheld devices — gives the authorities a powerful tool to track suspects.

In September, a man in Butte, Mont., pleaded guilty to rape after a
judge ruled that evidence from the global positioning unit in his car could
be used against him at trial. Prosecutors planned to use it to show that
the man, Brian D. Adolf, “prowled” in the town looking for a victim.

In New Brighton, Pa., a trucker’s system led the police to charge him
with setting his own home on fire. The system’s records showed his rig
was parked about 100 yards from his house at the time of the fire.

Critics, however, say the police should be allowed to acquire global
positioning data only by getting a warrant. Renée Hutchins, a University
of Maryland law professor, wrote an article recently suggesting Global
Positioning System data was protected under the Fourth Amendment.

“I think that in the last couple of years,” Ms. Hutchins said, “people
are starting to be aware that if they have these units in their car, people
can keep track of you. I think it’s a growing public awareness. The problem
is that most people feel like, ‘I’m not doing anything wrong, so who cares?’
But I think that’s the wrong way of looking at it.”

Developed for the military, the navigation devices started showing up
in cars in the 1990s. Prices have dropped sharply in the past few years,
and many units cost less than $150. The Consumer Electronics Association
estimates that 20 percent of American households own a portable Global
Positioning System unit and that 9 percent have vehicles equipped with
in-dash systems.